In the related art magnetic recording technology such as hard disk drives, a head is equipped with a reader and a writer. The reader and writer have separate functions and operate independently of one another, with no interaction therebetween.
FIGS. 1(a) and (b) illustrate related art magnetic recording schemes. A recording medium 1 having a plurality of bits 3 and a track width 5 has a magnetization parallel to the plane of the recording media. As a result, a magnetic flux is generated at the boundaries between the bits 3. This is also commonly referred to as “longitudinal magnetic recording”.
Information is written to the recording medium 1 by an inductive write element 9, and data is read from the recording medium 1 by a read element 11. A write current 17 is supplied to the inductive write element 9, and a read current is supplied to the read element 11.
The read element 11 is a sensor that operates by sensing the resistance change as the sensor magnetization direction change from one direction to the other. A shield 13 is also provided to reduce the undesirable magnetic fields coming from the media and prevent the undesired flux of adjacent bits from interfering with the one of the bits 3 that is currently being read by the read element 11.
In the foregoing related art scheme, the area density of the recording medium 1 has increased substantially over the past few years, and is expected to continue to increase substantially over the next few years. Correspondingly, the bit density and track density is expected to increase. As a result, the related art reader must be able to read this data having increased density at a higher efficiency and speed.
Due to these requirements, another related art magnetic recording scheme has been developed, as shown in FIG. 1(b). In this related art scheme, the direction of magnetization 19 of the recording medium 1 is perpendicular to the plane of the recording medium. This is also known as “perpendicular magnetic recording”. This design provides more compact and stable recorded data.
FIGS. 2(a)-(c) illustrate various related art read elements for the above-described magnetic recording scheme, known as “spin valves”. In the bottom type spin valve illustrated in FIG. 2(a), a free layer 21 operates as a sensor to read the recorded data from the recording medium 1. A spacer 23 is positioned between the free layer 21 and a pinned layer 25. On the other side of the pinned layer 25, there is an anti-ferromagnetic (AFM) layer 27.
In the top type spin valve illustrated in FIG. 2(b), the position of the layers is reversed. The operation of the related art spin valves illustrated in FIGS. 2(a)-(b) is substantially similar, and is described in greater detail below.
The direction of magnetization in the pinned layer 25 is fixed, whereas the direction of magnetization in the free layer 21 can be changed, for example (but not by way of limitation) depending on the effect of an external field, such as the recording medium 1.
When the external field (flux) is applied to a reader, the magnetization of the free layer 21 is altered, or rotated, by an angle. When the flux is positive the magnetization of the free layer is rotated upward, and when the flux is negative the magnetization of the free layer is rotated downward. Further, if the applied external field changes the free layer 21 magnetization direction to be aligned in the same way than pinned layer 25, then the resistance between the layers is low, and electrons can more easily migrate between those layers 21, 25.
However, when the free layer 21 has a magnetization direction opposite to that of the pinned layer 25, the resistance between the layers is high. This is because it is more difficult for electrons to migrate between the layers 21, 25.
Similar to the external field, the AFM layer 27 provides an exchange coupling and keeps the magnetization of pinned layer 25 fixed. The properties of the AFM layer 27 are due to the nature of the materials therein. In the related art, the AFM layer 27 is usually PtMn or IrMn
The resistance change when the layers 21, 25 are parallel and anti-parallel called ΔR should be high to have a highly sensitive reader. As head size decreases, the sensitivity of the reader becomes increasingly important, especially when the magnitude of the media flux is decreased. Thus, there is a need for high resistance change ΔR between the layers 21, 25 of the related art spin valve.
A summary of the well-known concepts of the related art spin valves is provided herein. When a polarized electron meets a ferromagnetic film, the electron is harmed by the magnetic moments and scattered. The lost of electron energy is transferred to the magnetic moment, based on the law of conservation of energy. This transfer of energy is manifested as torque, which acts on the ferromagnetic film. As noted above, the magnetization of the free layer may be perturbed and even switch under certain conditions, such as high current density, low magnetization, thin magnetic film and other intrinsic parameters, including exchange stiffness, and damping factor.
In the above-described related art spin valve, when the magnetic film has a sufficiently small magnetization, the resistance of its magnetization to energy transfer (momentum transfer) is weak, and its magnetization direction can be changed. Further, when the exchange stiffness (exchange energy between a magnetic moment and its neighbor) is small, some moments will switch before others.
Additionally, the related art spin valves under go a spin transfer effect. The spin transfer effect is similar to an additional damping effect added to the intrinsic damping factor (precession). But spin transfer has a different behavior than the intrinsic precession.
For a CPP-GMR spin valve with a current flowing through the film thickness, the pinned layer acts as a polarizing layer (source of polarization) because its magnetization does not change due to the flux of the recording media. However, the free layer is strongly affected by electron spin transfer, which exerts a torque on its magnetization. Spin transfer torque is calculated as follows:Spin transfer torque=τ−1[M×(M×P)],  (1)where τ is time constant depends on the current density and film intrinsic properties, and P is electron polarization direction, which depends on the source of polarization magnetization direction, in this case the pinned layer. FIG. 2(c) illustrates a related art dual type spin valve. Layers 21 through 25 are substantially the same as described above with respect to FIGS. 2(a)-(b). However, an additional spacer 29 is provided on the other side of the free layer 21, upon which a second pinned layer 31 and a second AFM layer 33 are positioned. The dual type spin valve operates according to the same principle as described above with respect to FIGS. 2(a)-(b). However, an extra signal provided by the second pinned layer 31 increases the resistance change ΔR.
FIG. 6 graphically shows the foregoing principle in the case of the related art longitudinal magnetic recording scheme as illustrated in FIG. 1(a). As the sensor moves across the recording media, the flux of the recording media at the boundary between bits, as shielded with respect to adjacent bits, provides the flux to the free layer, which acts according to the related art spin valve principles, which are described in greater detail below.
The operation of the related art spin valve is now described in greater detail. In the recording media 1, flux is generated based on polarity of adjacent bits. If two adjoining bits have negative polarity at their boundary, the flux will be negative. On the other hand, if both of the bits have positive polarity at the boundary, the flux will be positive. The magnitude of flux determines the angle of magnetization between the free layer and the pinned layer.
In addition to the foregoing related art spin valve in which the pinned layer is a single layer, FIG. 3 illustrates a related art synthetic spin valve. The free layer 21, the spacer 23 and the AFM layer 27 are substantially the same as described above. In this figure only one state of the free layer is illustrated. However, the pinned layer further includes a first sublayer 35 separated from a second sublayer 37 by a spacer 39.
In the related art synthetic spin valve, the first sublayer 35 operates according to the above-described principle with respect to the pinned layer 25. Additionally, the second sublayer 37 has an opposite spin state with respect to the first sublayer 35. As a result, the pinned layer total moment is reduced due to anti-ferromagnetic coupling between the first sublayer 35 and the second sublayer 37. A synthetic spin valve head has a pinned layer with a total magnetic flux close to zero and thus greater stability and high pinning field can be achieved.
FIG. 4 illustrates the related art synthetic spin valve with a shielding structure. As noted above, it is important to avoid unintended magnetic flux from adjacent bits from being sensed during the reading of a given bit. A protective layer 41 is provided on an upper surface of the free layer 21 to protect the spin valve against oxidation before deposition of top shield 43, which is usually done by electroplating in separated system. Similarly, a bottom shield 45 is provided on a lower surface of the AFM layer 27. A buffer layer, not shown in FIG. 4, is usually deposited before AFM layer 27 for a good spin-valve growth. The effect of the shield system is shown in FIG. 6, as discussed above.
As shown in FIGS. 5(a)-(d), there are four related art types of spin valves. The type of spin valve structurally varies based on the structure of the spacer 23.
The related art spin valve illustrated in FIG. 5(a) uses the spacer 23 as a conductor, and is used for the related art CIP scheme illustrated in FIG. 1(a), for a giant magnetoresistance (GMR) type spin valve. Because, the direction of sensing current magnetization, as represented by “i”, is in the plane of the GMR element.
In the related art GMR spin valve, resistance is minimized when the magnetization directions (or spin states) of the free layer 21 and the pinned layer 25 are parallel, and is maximized when the magnetization directions are opposite. As noted above, the free layer 21 has a magnetization which direction can be changed. Thus, the GMR system avoids perturbation of the head output signal by minimizing the undesired switching of the pinned layer magnetization.
GMR depends on the degree of spin polarization of the pinned and free layers, and the angle between their magnetic moments. Spin polarization depends on the difference between the spin state (up or down) in each of the free and pinned layers. These concepts are discussed in greater detail below.
The GMR scheme will now be discussed in greater detail. As the free layer 21 receives the flux that signifies bit transition, the free layer magnetization rotates by a small angle in one direction or the other, depending on the direction of flux. The change in resistance between the pinned layer 25 and the free layer 21 is proportional to angle between the moments of the free layer 21 and the pinned layer 25. There is a relationship between resistance change and efficiency of the reader.
The GMR spin valve has various requirements. For example, but not by way of limitation, a large resistance change ΔR is required to generate a high output signal. Further, low coercivity is desired, so that small media fields can also be detected. With high pinning field strength, the AFM structure is well defined, and when the interlayer coupling is low, the sensing layer is not adversely affected by the pinned layer. Further, low magnetistriction is desired to minimize stress on the free layer.
However, the foregoing related art CIP-GMR has various disadvantages. One of them is that the electrode connected to the free layer must be reduced in size which will cause an overheat and damage of the head. Also the readout signal available from CIP-GMR is proportional to the MR head width, that means there is a limitation for CIP-GMR at high recording density.
To overcome the foregoing related art problem related art magnetic recording schemes are using CPP-GMR head, where the sensing current flows in the direction perpendicular to the spin valve plane. As a result, size can be reduced and thermal stability can be increased. Various related art spin valves that operate in the CPP scheme are illustrated in FIGS. 5(b)-(d), and are discussed in greater detail below.
FIG. 5(b) illustrates a related art tunneling magnetoresistive (TMR) spin valve for CPP scheme. In the TMR spin valve, the spacer 23 acts as an insulator, or tunnel barrier layer. Thus, the electrons can cross the insulating spacer 23 from free layer to pinned layer or verse versa. TMR spin valves have an increased MR on the order of about 30-50%.
FIG. 5(c) illustrates a related art CPP-GMR spin valve. While the general concept of GMR is similar to that described above with respect to CIP-GMR, the current is transferred perpendicular to the plane, instead of in-plane. As a result, the difference in resistance, as well as the intrinsic MR, are substantially higher than the CIP-GMR.
In the related art CPP-GMR spin valve, there is a need for a large resistance change ΔR, and a moderate element resistance for having a high frequency response. A low coercivity is also required so that a small media field can be detected. The pinning field should also have a high strength. Additional details of the CPP-GMR spin valve are discussed in greater detail below.
FIG. 5(d) illustrates the related art ballistic magnetoresistance (BMR) spin valve. In the spacer 23, which operates as an insulator, a ferromagnetic region 47 connects the pinned layer 25 to the free layer 21. The area of contact is on the order of a few nanometers. As a result, there is a substantially high MR, due to electrons scattering at the domain wall created within this nanocontact. Other factors include the spin polarization of the ferromagnets, and the structure of the domain that is in nano-contact with the BMR spin valve.
However, the related art BMR spin valve is in early development, and is not in commercial use. Further, there are related art problems with the BMR spin valve in that nano-contact shape and size controllability and stability of the domain wall must be further developed. Additionally, the repeatability of the BMR technology is yet to be shown for high reliability.
In the foregoing related art spin valves of FIGS. 5(a)-(d), the spacer 23 of the spin valve is an insulator for TMR, a conductor for GMR, and an insulator having a magnetic nano-sized connector for BMR. While related art TMR spacers are generally made of more insulating metals such as alumina, related art GMR spacers are generally made of more conductive metals, such as copper.
FIGS. 7(a)-(b) illustrate the structural difference between the CIP and CPP GMR spin valves. As shown in FIG. 7(a), there is a hard bias 998 on the sides of the GMR spin valve, with an electrode 999 on upper surfaces of the GMR. Gaps 997 are also required. As shown in FIG. 7(b), in the CPP-GMR spin valve, an insulator 1000 is deposited at the side of the spin valve that the sensing current can only flow in the film thickness direction. Further, no gap is needed in the CPP-GMR spin valve.
As a result, the current has a much larger surface through which to flow, and the shield also serves as an electrode. Hence, the overheating issue is substantially addressed.
Further, the spin polarization of the layers of the spin valve is intrinsically related to the electronic structure of the material, and a highly resistive material can induce an increase in the resistance change. Accordingly, there is an unmet need for a material having the necessary properties and thickness for operation in a CPP-GMR system.
Additional factors associated with the performance of the related art CPP-GMR system are provided below. Various related art studies have demonstrated the effect of electron spin polarized on magnetization switching, including M. Tsoi et al., Phys. Review Letters, 80, 4281 (1998), J. C. Slonczewski, J. Magnetism and Magnetic Materials, 195, L261 (1999), J. A. Katine et al., Phys. Review Letters, 84, 3149 (2000), M. R. Pufall et al., Applied Physics Letters, 83 (2), 323 (2003), the contents of which are incorporated herein by reference.
Spin transfer can occur by polarized electron which interact with the magnetization of the ferromagnetic (FM) layer. The electron becomes polarized when it travels across the pinned FM layer (source of polarization).
The following equation expresses the minimum current ISW required to switch the magnetization of the FM layer by spin transfer effect:
                              I          sw                =                                            et              F                        ℏɛ                    ⁡                      [                                                            23.4                  ⁢                                      M                    S                                    ⁢                  D                                                  2                  ⁢                  ℏγ                                            +                              6.3                ⁢                                  r                  2                                ⁢                α                ⁢                                                                  ⁢                                                      M                    S                                    ⁡                                      (                                                                  H                        app                                            +                                              H                        exch                                            -                                              M                        S                                                              )                                                                        ]                                              (        2        )            
In equation (2), e is the charge of electron, ℏ is Planck constant, tF is the free layer thickness, MS is the saturation magnetization of the free layer, r is the contact area, γ is the gyromagnetic factor, D is the exchange stiffness, α is the damping factor, Happ is the applied field, Hexc is the effective interlayer coupling field.
To reduce the undesired effect of spin transfer on switching the free layer magnetization (in other words, to maximize ISW), materials with high MS, D, and α are required. Also, thicker films are more stable against spin transfer induced switching (instability).
Regarding the free layer saturation magnetization MS, a small MS value is needed in the free layer, because the sensitivity of the head (output signal) is reduced as MS increases. More specifically, rotation of the free layer magnetization direction is more difficult for a large MS and easier for a small MS.
The contact size r (assumed to be circular for simplicity) also affects ISW. However, the head size is related to r and is thus continuously being reduced to achieve higher recording density.
In the related art studies, correlation between intrinsic properties and spin transfer switching has been determined. Also, dynamic response of magnetization switching has been studied. In conclusion, the ability of the head (sensor) to engage in fast switching of magnetization at a high frequency (e.g., GigaHertz) is important for high-speed reading of the recorded information (high data rate).
As recording media bit size is reduced, a thinner free layer is also needed. In the related art, there is a need for a free layer with a thickness of less than 3 nm for a sensor having a recording density of about 150 GB per square inch. In the future, it is believed that the need to reduce free layer thickness will continue. There is also a need to sense increasingly smaller bits at a very high frequency (i.e., high data rate) in recording head reader technology.
However, there are various problems and disadvantages in the related art. For example, but not by way of limitation, there is a problem of thermal stability, due to a high demagnetizing field. Further, there is a vortex effect, due to magnetic fluctuation, mainly at the edge.
Additionally, there is a spin transfer effect, due at least in part to transfer of electron momentum to the magnetic moment of the ferromagnetic thin film. If the current density is too high, the free layer magnetization will switch (i.e., the spin transfer occurs). As a result, there is a perturbation of the magnetic thin film magnetization, at least partially (in which case there is non-uniformity) or completely (in which case there is a complete switch of direction).
Accordingly, there is a related art need to minimize this spin transfer effect, such that the free layer magnetization is affected only by the media flux.
Also, perturbation or fluctuation of the magnetic moment distribution in the free layer (i.e., sensor) generates an additional source of noise. As a result, the signal noise ratio is reduced.